Structural and magnetic ordering in La0.6Ca0.4MnO3

The structural and magnetic ordering in La_0.6Ca_0.4MnO₃ has been studied by neutron powder diffraction as a function of temperature between 15 and 300 K. The para-ferromagnetic transition at T∼250 K is accompanied by significant structural distortions in the form of octahedral Mn–O₆ rotations. At 15 K, the total refined ferromagnetic moment on the Mn site was obtained as 3.1 μ_B, in reasonable agreement with the total expected average moment of mixed Mn³⁺/Mn⁴⁺ matrix.     

Critical behavior and magnetic relaxation dynamics of Nd0.4Sr0.6MnO3 nanoparticles

Detailed DC and AC magnetic properties of chemically synthesized Nd_0.4Sr_0.6MnO₃ with different particle size (down to 27?nm) have been studied in details. We have found ferromagnetic state in the nanoparticles, whereas the bulk Nd_0.4Sr_0.6MnO₃ is known to be an A-type antiferromagnet. A Griffiths-like phase has also been identified in the nanoparticles. Further, critical behaviour of the nanoparticles has been studied around the second-order ferromagnetic-paramagnetic transition region (|(T?T _C)/T _C|???0.04) in terms of modified Arrott plot, Kouvel–Fisher plot and critical isotherm analysis. The estimated critical exponents (β, γ, δ) are quite different from those predicted according to three-dimensional mean-field, Heisenberg and Ising models. This signifies a quite unusual nature of the size-induced ferromagnetic state in Nd_0.4Sr_0.6MnO₃. The nanoparticles are found to be interacting and do not behave like ideal superparamagnet. Interestingly, we find spin glass like slow relaxation of magnetization, aging and memory effect in the nanometric samples. These phenomena have been attributed to very broad distribution of relaxation time as well as to inter-particle interaction. Experimentally, we have found out that the dynamics of the nanoparticle systems can be best described by hierarchical model of spin glasses.     

Transport and magnetic properties of La1.48Nd0.4Sr0.12CuO4/La0.67Sr0.33MnO3 bilayer

We have grown La_1.48Nd_0.4Sr_0.12CuO₄/La_0.67Sr_0.33MnO₃ (LNSCO/LSMO) bilayer structure on SrTiO₃ (0 0 1) substrate. Both temperature dependences of resistivity and magnetization curves show anomalies between 60 < T < 80 K, where a low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) structural transition is observed in LNSCO bulk crystal. It is suggested that the formation of domains in LSMO layer can relax the strains caused by the LTO–LTT transition in LNSCO layer.     

Mössbauer study on the magnetic field-induced insulator-to-metal transition in perovskite Eu0.6Sr0.4MnO3

We have investigated the spin dynamics of a distorted perovskite Eu_0.6Sr_0.4MnO₃ by means of Mössbauer spectroscopy. Below 70 K the exchange interaction grows gradually, and below 42 K the spins turn into a cluster glass state. The magnetic field-induced insulator-to-metal (IM) transition at low temperature is a transition from cluster glass to ferromagnet. The induced metallic phase seems to be still in non-uniform electronic state. On the other hand, at 80 K, just above T _c of the induced ferromagnet, a metamagnetic transition was observed.     

Study of the elastic constants in a La0.6Sr0.4MnO3 film by means of laser-generated ultrasonic wave method

Two dimensional (2D) displacement field of the laser-generated ultrasonic wave is detected in a two-layered structure of La(0.6)Sr(0.4)MnO(3) (LSMO)/MgO system by means of the optical difference detection method. In order to obtain the elastic constants of the La(0.6)Sr(0.4)MnO(3) thin film, the displacement field of the laser-generated ultrasonic wave for the La(0.6)Sr(0.4)MnO(3)/MgO system is analyzed with finite element method (FEM). We further compare the theoretical simulations with the experimental results, and the elastic constants for the LSMO film, i.e., the Young's modulus and Poisson ratio, are obtained.     

（La0．6Nd0．4）0．7Sr0．3MnO3薄膜的厚度依赖的结构与输运性

厚度为6～100 nm的(La0.6Nd0.4)0.7,Sr0.3MnO3(LNSMO)薄膜是由脉冲激光沉积生长在SrTiO3[STO(001)]衬底上.X-射线衍射(XRD)线扫描、ω-摇摆曲线和倒空间衍射显示,随着膜厚(t)的变化,薄膜的结构经历了由完全应变到部分弛豫的变化.电阻测量显示存在两个不同的厚度依赖的金属-绝缘体转变温度(Tp),对于应变的薄膜(t≤17 nm),Tp敏感地依赖于双轴应变和膜厚;而对较厚的薄膜(t≥35nm),Tp对于膜厚是较小敏感的.这个厚度依 赖的输运性的变化可被解释为菱形LNSMO的角度畸变诱导的应变和弛豫.我们的结果表明金属-绝缘体转变温度和相分离都受薄膜的应变态控制.     

Giant magnetocaloric effect near the curie temperature in the Sm0.6Sr0.4MnO3 manganite

A method is proposed for the calculation of the magnetocaloric effect from simultaneous measurements of thermal expansion and magnetostriction made in different regimes (adiabatic and isothermal). The magnitude of the magnetocaloric effect for Sm_0.6Sr_0.4MnO₃ is estimated. It is found that near the Curie temperature T _C it passes through a maximum to reach a giant value ΔT=4.6 K for ΔB=0.84 T. In addition, in the neighborhood of T _C, we observed colossal magnetoresistance Δρ/ρ = [ρ(H) ? ρ(0)]/ρ(0) = 72% in a weak magnetic field of 0.84 T, a giant negative volume magnetostriction ω=?5×10^?4 in a field of the same strength, and a large change in the sample volume ΔV/V ≈ 0.1%.     

Zero-field cooled exchange bias and magnetization reversal in La1.5Sr0.5Co0.4Fe0.6MnO6

The polycrystalline sample La_1.5Sr_0.5Co_0.4Fe_0.6MnO₆ (LSCFMO) was prepared by sol-gel method and its magnetic properties were studied. The interesting magnetization reversal phenomenon and the zero-field cooled exchange bias (ZEB) effect were simultaneously observed in LSCFMO. ZEB effect can exist in a wider temperature range (0–200 K) compared with La_1.5Sr_0.5CoMnO₆ (0–10 K), which is very important in the potential applications. A schematic diagram based on the coupling between the Fe³⁺ spins, Mn³⁺ spins and Co²⁺ or Co³⁺ spins is used to understand the ZEB and the reversal behaviors. Due to the doping of 60% Fe ion, the magnetic microstructure of LSCMFO is more complex than that of LSCMO, resulting in the meta-stable spin structure and more interesting magnetic phenomenon.     

Dielectric relaxations,magnetodielectric and magnetoelectric interactions in Bi0.6La0.4MnO3 ceramics

Complex permittivity ε*/ε₀ = ε′/ε₀–iε″/ε₀ of the bismuth–lanthanum manganite Bi_0.6La_0.4MnO₃ ceramics has been measured in the temperature range of 10–220 K at frequencies f = 20–10⁶ Hz and magnetic inductions B = 0–0.846 T. At a temperature of 80 K, the spectra ε′/ε₀(t) and ε″/ε₀(t) demonstrate the dielectric relaxation that is a superposition of contributions of several relaxation processes, each of which is dominant in its frequency range: I (f < 10³ Hz, II (10³ < f < 10⁵ Hz), and III (10⁵ < f < 10⁶ Hz). In the range of 10–120 K, anomalous behavior of ε′/ε₀(T) and ε″/ε₀(T) is observed near the temperature of the transition from the paramagnetic to ferromagnetic phase and is due to the Anderson localization of charge carrier on a spin disorder.     

Electrode reaction and microstructure of La0.6Sr0.4CoO3−δ thin films

Dense La_0.6Sr_0.4CoO_3−δ film electrodes were deposited by pulsed laser deposition (PLD) on Ce_0.9Gd_0.1O_1.95 electrolytes. The grain size of one film was 300–500 nm, and the other was 30–50 nm. DC polarization and AC impedance measurements were performed at 873 K–1073 K in O₂–Ar gas mixtures. From investigations of the electrochemical capacitances, the rate determining process for both electrodes were confirmed to be the surface reaction. The analyses in the electrochemical resistance revealed that the oxygen adsorption/desorption rate was faster on the electrode with smaller grain size. DC responses agreed with AC results, so the current density on the nano-grain electrode was larger by half an order than those of the sub-micron-grain electrode. Under a dilute oxygen atmosphere, the rate determining step transferred from a surface reaction to a gas phase diffusion.     

Nonlinear optical spectroscopy of (La0.6Pr0.4)0.7Ca0.3MnO3 manganite

The electronic structure of doped orthorhombic (La_0.6Pr_0.4)_0.7Ca_0.3MnO₃ manganite is studied using the second optical harmonic generation method. The analysis of the frequency and polarization dependence of the intensity related to the second harmonic demonstrates that the applied magnetic field stimulates a transition of spin-polarized electrons from the d orbitals of Mn ions to the p orbitals of O₂ ligands. The manifestation of such transition in the orthorhombic RMnO₃ manganite with the distorted crystal lattice is attributed to the dipole-forbidden electronic transitions arising in the applied magnetic field. The mechanism of magnetooptical activity induced by the charge transfer between Mn and O ions is discussed.     

La0.4Ca0.6MnO3系统中Mn位Fe和Cr掺杂效应的比较性研究

利用固相反应法制备了Mn位Fe³⁺和Cr³⁺掺杂的系列锰氧化物La_0.4Ca_0.6(Mn_1-x(y)B_x(y))O₃ (B=Fe³⁺ (0≤x≤0.1); Cr³⁺ (0≤y≤0.1)) 多晶样品,研究了掺杂对样品输运性质的影响.实验结果表明两种离子具有截 关键词： 锰氧化物 反铁磁 磁电阻效应     

Laser patterning and magnetic properties of perovskite-type La0.7Sr0.3MnO3 crystals on the glass surface

The laser-induced crystallization technique has been applied to 10La₂O₃-35SrO-25MnO₂-30B₂O₃ glass (mol%) in order to examine the possibility of the formation and morphology control of perovskite-type La_1?xSr_xMnO₃ crystals on the glass surface. It is found from X-ray diffraction analyses that La_0.7Sr_0.3MnO₃ crystals are formed in the sample obtained by continuous wave Y b:Y V O₄ fiber laser (wavelength: 1080 nm) irradiations (power: 350 mW, scanning speed: 10 μm/s). La_0.7Sr_0.3MnO₃ manganites in the laser-irradiated samples are proposed to be ferromagnetic crystals having a Curie temperature of ～315 K from magnetization measurements and the Arrott plot. The surface morphology of laser-irradiated parts is not smooth, and La_0.7Sr_0.3MnO₃ crystals are present randomly without any orientations. It is clarified that Mn⁴⁺ ions in the glass act as suitable transition metal ions for the laser-induced crystallization.     

Nd0.5Sr0.4Pb0.1MnO3的结构和磁性

通过室温下的中子衍射和磁性测量对多晶样品Nd0.5Sr0.4Pb0.1MnO3的结构和磁性进行了实验研究.中子衍射结果表明,该样品具有正交的钙钛矿结构,空间群是Pnma,即结构发生了晶场畸变.由M-T和R-T曲线可知,居里温度TC=273 K,其特征是随着温度的增加样品经历了从铁磁金属态转变到顺磁半导态,且转变温度Tp＝225 K;用锰氧化物晶场和双交换作用的竞争解释了其温度Tp以下的金属特性.     

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La_0.9Sr_0.1MnO₃ compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q ₂ mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 41, 1064–1069 (June 1999)     

In-situ Mössbauer spectroscopy and thermogravimetry of La0.2Sr0.8FeO3−Δ and La0.4Sr0.6FeO3−Δ

Single phase perovskite-type oxides La_0.2Sr_0.8FeO_3−Δ (LSF28) and La_0.4Sr_0.6FeO_3−Δ (LSF46) were prepared by the glycine nitrate process. The oxygen nonstoichiometry of LSF46 was studied by precision thermogravimetry at 700 and 900 °C as a function of oxygen partial pressure. Mössbauer spectra were measured at room temperature, as well as at 500 and 700 °C (LSF28) and at 700 and 900 °C (LSF46) under varying oxygen partial pressures. Good agreement is observed for the pO₂-dependent nonstoichiometry of LSF46 at 700 and 900 °C as derived from thermogravimetry and in-situ Mössbauer spectroscopy. At low pO₂, the Mössbauer isomer shifts and quadrupole splittings indicate a phase transition in LSF46 at 700 °C and in LSF28 at the two temperatures studied from a disordered perovskite phase to a brownmillerite-like structure with larger quadrupole splitting.     

Defects and the structure of a manganite La0.85Sr0.15MnO3 crystal

Defects in a ferromagnetic crystal of manganite La_0.85Sr_0.15MnO₃ were created by irradiation with fast neutrons (E > 0.1 MeV). Fast neutrons produce defect clusters in a crystal lattice. The volume fraction of the clusters in the crystal after irradiation to a dose F = 2 × 10¹⁹ cm^?2 (T _irr = 340 K) was ?40%. The structural and magnetic states of the modified manganite were studied using thermal-neutron diffraction and magnetic measurements. It was revealed that neutron irradiation of a crystal suppresses the cooperative Jahn-Teller effect and the initial charge modes and decreases the temperature of ferromagnetic ordering. Under irradiation with fast neutrons, the crystalline structure of the manganite changes from the orthorhombic O′ to the pseudocubic O* phase. Arguments are advanced in favor of the specific features of the irradiated-manganite structural state being determined by long-wavelength strains induced in the crystal by antisite defects.     

Total-reflection inelastic X-ray scattering from a 10-nm thick La0.6Sr0.4CoO3 thin film

To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total-reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5 edge with momentum transfer as an example.     

Structural,electrical, magnetic,elastic and anelastic behaviour of Nd0.6Ca0.4MnO3

With a view to understanding the structural, electrical and magnetic, elastic and anelastic behaviour of charge ordered Nd_0.6Ca_0.4MnO₃, systematic investigations were undertaken. The sample was synthesized by a sol–gel route. The sample was characterized by X-ray diffraction (XRD) using a Rietveld refinement technique and was found to have orthorhombic structure with space group Pnma. A study on the variation of electrical resistivity with temperature has been carried out in the range 100–300 K and it was found to exhibit a field induced transition. The ac susceptibility studies show two transition temperatures, which are attributed to charge ordering and a Néel transition. Internal friction and longitudinal modulus studies were carried out using the composite oscillator technique. An effort has been made to explain the observed anomalous behaviour by a qualitative model.     

Anomalous magnetic field dependence of T AF in La 0.95 Sr 0.05 MnO 3

The magneto-elastic properties of single-crystalline La_0.95Sr_0.05MnO₃ have been studied ultrasonically. Our investigations focussed on the temperature interval where magnetic ordering starts to evolve and results in a spin canted antiferromagnetic ground state. In detail the experiments revealed that the magnetic order parameter in low-doped manganite is only weakly coupled to lattice strains. Furthermore, the anomalous temperature dependence of the order parameter as found resembles highly that in stoichiometric LaMnO₃. However, the main and most surprising finding is that external magnetic fields favor the spin canted phase in La_0.95Sr_0.05MnO₃. It is unclear at present how the exchange interaction can be tuned by magnetic fields in the way observed and we are not aware of existing theoretical concepts which might give a plausible explanation for the unexpected field dependent behavior of the critical temperature. We believe, however, that this behavior primarily results from the fact that the exchange interaction depends sensitively on the orbital configuration of the manganese d electrons. Received 27 March 2000